1. Field of the Invention
Embodiments of the invention relate to electro-deionization methods and apparatus.
2. Background of the Related Art
An Electro-Deionization apparatus (EDI) is a device that removes the dissolved impurities of reverse osmosis (“RO”) permeate water with the help of resin media, ion exchange membrane and DC current. The EDI process is a continuous process as it does not need chemicals such as acid and caustic for regeneration of resin media and membranes. The resin media is regenerated during the purification of water due to continuous water splitting occurring inside the EDI apparatus. Splitting of H+ and OH− ions happens due to the electric potential generated within the dilute compartment from the H2O molecules which regenerates the corresponding resin ions continuously.
An EDI apparatus is generally used for the purification of reverse osmosis permeate water up to a level of 0.055 μS/cm conductivity and makes it suitable for various industrial applications including but not limited to boiler feed/steam generation, microelectronics/semiconductor makeup or rinse water, and purified USP grade water.
An EDI apparatus is typically made up of ion exchange resin, ion exchange membranes, electrodes for DC supply and hardware components for water flow distribution. The arrangement of ion exchange membranes, anion exchange membranes and cation exchange membrane, are very important in an EDI apparatus. They are generally arranged in alternate manner with respect to anode and cathode electrodes. The ion exchange resin media are filled in the chambers/compartments which are formed due the arrangement of anion and cation exchange membranes. This leads to the formation of dilute chambers and concentrate chambers.
The dilute chambers are those in which the feed water (RO permeate) gets purified and become ultra pure water whereas the concentrate chambers are adjacent alternate positioned to dilute in which the removed ions from the dilute chambers are collected and flushed out from the apparatus with the help of separate water stream. A small portion of the water is also used for flushing and cooling the cathode and anode electrodes, called Electrode Rinse stream and the chamber is called the Electrode Rinse Chamber.
The efficiency and commercial utilization of any EDI apparatus depends upon the quantity of product water produced per unit area of membrane, per unit volume of ion exchange media or per cell pair of membranes. There are many commercial EDI apparatus are available in market which can be easily divided into three categories—
1) Low Flow Rate EDI Apparatus:
This type of EDI apparatus are generally thin cell plate and frame EDI having product flow rate range of 1.5 m3/hr and 2.0 m3/hr with 30 or more cell pair. The output product flow of each dilute chamber is generally in the range of 50 to 60 LPH or less.
2) Medium Flow Rate EDI Apparatus:
The second category EDI apparatus are generally thick cell plate and frame EDI having product flow rate of 2.5 m3/hr-5.0 m3/hr with 30 cell pair or more. The output product flow of each dilute chamber is generally in the range of 80 to 170 LPH or less.
In both the first and the second category the pressure drop across dilute chamber (feed to product) is typically around 20 to 30 psi for nominal flow rate. For maximum flow rate it typically increases up to 40-60 psi. The effective length of dilute chambers of such EDI apparatus vary from 350 mm to 450 mm and width vary from 100 mm to 200 mm. The resin volume inside the dilute chambers of these EDI apparatus does not allow increase in the flow rate due to high pressure drops, mechanical leaks or mechanical strength of the apparatus.
The typical flow configuration of both category EDI apparatus is shown in FIG. 1. A third category of EDI apparatus is also used.
3) High Flow EDI Apparatus:
To achieve high product flow rate, more than 5.0 m3/hr, a third category of EDI apparatus is also used, which is generally the combination of multiple EDI apparatus, connected together in parallel with 40 to 60 number of cell pairs per unit, for producing high product flow rate from the stacks. The product flow rate per dilute chamber of this type EDI apparatus is similar to medium flow rate EDI apparatus. The high product flow through these EDI apparatus is only due to the increased area of dilute chambers.
For the production of ultra pure water, an EDI apparatus generally is used to purify either permeate water of single pass RO, where the feed ion load is high with challenging scaling ions, or permeate water of second pass RO permeate, where the feed ions load are very less with negligible amount of scaling ions.
The scaling ions (like Ca2+, Mg2+, CO3, SiO2, etc) have big role to play in any EDI operation and have been the cause for limiting conditions requiring additional pretreatment that may be uneconomical in many cases. Some solutions to this problem have been proposed. For example, the fractional deionization process reported in U.S. Pat. No. 6,896,814, incorporated by reference herein, uses a dual voltage process for the removal of higher load of scaling ions without scaling in EDI apparatus.
EDI design for certain flow depends upon the feed condition and the product quality requirement. For harsher feed condition such as single pass RO permeate water with challenging scaling ions, the product flow is typically reduced. This makes the system costly and therefore unattractive for use.
The typical product flow rate of an EDI apparatus at different feed hardness (as CaCO3) and feed conductivity equivalent (FCE) loads are summarized as,                a. Product flow rate=2.4 m3/hr when feed hardness (as CaCO3) is 3 ppm and total FCE load is 25-30 μS/cm for the product quality requirement of more than 10 to 16 MOhms        b. Product flow rate=3.5 m3/hr when feed hardness (as CaCO3) is 1 ppm and total FCE load is 15-20 μS/cm for the product quality requirement of more than 10 to 16 MOhms        c. Product flow rate=4.5 m3/hr when feed hardness (as CaCO3) is 0.1 ppm and total FCE load is <10 μS/cm for the product quality requirement of more than 10 to 16 MOhms        
When the feed hardness (as CaCO3) is less than 0.1 ppm like in second pass RO permeate water, the EDI behavior is quite different. Scaling is not a major concern, and the rate of effective in-situ media regeneration is the main criterion to govern the process and higher product quality can be easily achievable even with high flow rate but the main limitation is the higher pressure drop across the dilute chamber in regular flow mode.
The previously mentioned fractional deionization process of U.S. Pat. No. 6,896,814 B2 is a two stage process that deals with hardness and silica removal in separate zones because of their different current requirement. The design of an apparatus for this reason has two stages and is able to produce product flow rate up to 5.0 m3/hr with its regular flow mode. This process/apparatus when used for double pass RO grade water with novel split flow design is able to treat as high as 8 to 10 m3/hr of product water against normally product flow rate which is 3 to 5 m3/hr. Single stack with 8 to 10 m3/hr reduces the line connections, minimize pressure drop across dilute chamber, reduce power consumption per unit volume of water and makes a economical and viable proposition for the user.